regrout

I have a 70s-80s ceramic tile shower I need to regrout.The tile is in good shape but the grout is falling out in places and moldy in others. Only option for this job is to rake the joints as much as possible and regrout. I have worked with tile before and would normally take it down to framing but not happening here.

Is there a fitting for my dremel that will help?
Do I need to prep with a bonding agent pre grout or just clean or both?
thanks Timothy

Tp,
I believe dremel has a small guide that keeps the tool at an angle and lets you set the depth of the bit so you can cut a groove and keep it at the same depth. Also the oscillating fein-type tools have a blade for removing grout. I think it might be a little easier than the conventional dremel. I would clean everything when you are done grinding, then grout, then seal.
Mike Hawkins